I was just relieving some options for site improvements and one of the suggestions was to leverage browser cache more. Interestingly is that the only images that need this improvement are the custom images in the style folder but not any of the xenForo images default images. So I'm guessing xenForo already defines the cache expiration time for its images but I did not find anything in the .htaccess and I was wondering where that is so I can can the custom image to that list as well.
Those images are served by your web server and we don't set anything specific to it. You could set default expiration times using an .htaccess for example (conceptually: http://gtmetrix.com/leverage-browser-caching.html). Obviously you need to be careful changing the images then. You'll need to change the URL to them if you want to change them.